Is it possible to lock a bone in pose mode while removing the influence from the parent?
Possibly the most straightforward way to do what you want is to use a Child Of constraint. It works just like parenting, except that its effect can be keyframed, unlike actual parenting. So it can be turned on or off at will.
Begin by selecting your armature and then going into Edit Mode, then selecting the child bone in the chain you want to be pinned. Alt + p to bring up the Unparent menu, then Clear Parent. Now go into Pose Mode. The newly liberated child bone should still be selected. If not, select it. Go to the Bone Constraints Tab in the Properties Panel, and create a Child Of constraint from the Constraints dropdown. Specify the armature as the target for this constraint, then the parent bone, by name, also in the associated field (there are two text fields for the target when the target is part of an armature. The first text field is for the armature itself, and the second is for the bone within that armature which you specifically want to target). Now your child bone should move as a child while your parent is selected and rotated. When you are ready to turn off the constraint, look in the Constraint properties for the Influence slider. Right-click on it and set a keyframe. Do this on the frame just before you want it turned off. Advance the frame by one, and then slide the influence bar to zero. Then set another keyframe. Your child bone will now behave as if it is not parented.
This may or may not work ideally depending on whatever your exact situation is. But it definitely overcomes the problem of needing a parenting relationship on and off on the fly while animating.